This invention relates generally to the field of printing devices, and in particular to improving the quality of print yielded from continuous stream ink jet printers in which a liquid ink stream is broken into droplets, some of which are selectively deflected by a gas stream.
Traditionally, digitally controlled ink jet color printing is accomplished by one of two technologies. Both can utilize independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead and each channel includes a nozzle from which droplets of ink are selectively ejected and deposited upon a print medium, such as paper. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million shades or color combinations.
The first technology, commonly referred to as xe2x80x9cdrop on demandxe2x80x9d (DOD) ink jet printing, provides ink droplets for impact upon a recording surface using a pressurization actuator, such as a thermal actuator, piezoelectric actuator, or the like. Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle helping to keep the nozzle clean.
With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled. Some naturally occurring materials possessing these characteristics are quartz and tourmaline. The most commonly produced piezoelectric ceramics are lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
The second technology, commonly referred to as xe2x80x9ccontinuous streamxe2x80x9d or xe2x80x9ccontinuousxe2x80x9d inkjet printing, uses a pressurized ink source which produces a continuous stream of ink droplets. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When printing is desired, the ink droplets are deflected into an ink capturing mechanism and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism. Typically, continuous inkjet printing devices are faster than droplet on demand devices and can produce high quality printed images and graphics.
U.S. Pat. No. 1,941,001, issued to Hansell, and U.S. Pat. No. 3,373,437 issued to Sweet et al., each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as xe2x80x9cbinary deflectionxe2x80x9d continuous ink jet printing.
Continuous ink jet printers that utilize electrostatic charging devices and deflector plates require many components and large spatial volumes in which to operate. This results in continuous inkjet printheads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.
U.S. Pat. No. 3,709,432, issued to Robertson, discloses a method and apparatus for stimulating a filament of ink to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a print media.
U.S. Pat. No. 4,190,844, issued to Taylor, on Feb. 26, 1980, discloses a continuous inkjet printer in which a printhead supplies a filament of working fluid that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an xe2x80x9con/offxe2x80x9d or an xe2x80x9copen/closedxe2x80x9d type having a diaphram that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is to be printed or non-printed. The second pneumatic deflector is a continuous type having a diaphram that varies the amount a nozzle is open depending on a varying electrical signal received the central control unit. This oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time.
The use of an air flow to deflect droplets in a continuous inkjet printhead reduces the complexity of the printhead. However, such printheads are sensitive to environmental conditions and thus can produce inconsistent print quality.
An object of the present invention is to improve the quality of print from of a continuous ink jet printhead. To achieve this and other objects, a first aspect of the invention is an apparatus for printing an image comprising an ink droplet forming mechanism configured to selectively create a stream of ink droplets having a plurality of volumes and traveling along a trajectory path. A droplet deflector is configured to generate a gas flow at an output thereof interacting with the stream of ink droplets thereby separating ink droplets having one of a plurality of volumes from ink droplets having another of a plurality of volumes. A pressure sensor is positioned proximate the output and configured to generate a pressure indication signal. A controller is coupled to the pressure sensor and configured to output a compensation signal based on the indication signal, and a pressure mechanism is operatively coupled to the controller to adjust the gas flow generated by the droplet deflector.